What can forging be classified into according to the forming method?

Forging is a method of metal pressure processing, which is to heat the metal to a high temperature and change the shape of the metal by hitting the forging hammer or compressing the press. The forging method can obtain finished products or blanks for further machining.

Forging is a major form of pressure processing of mechanical parts. In the metallurgical industry, in addition to producing parts in the machine repair department, it is mainly used for the blanking of alloy steel ingots with poor plasticity and the production of square steel, round steel, and flat steel.

Forging can eliminate defects such as cast looseness produced by the metal in the smelting process, optimize the microstructure, and at the same time, because the complete metal flow line is preserved, the mechanical properties of forgings are generally better than those of castings of the same material. Important parts with high loads and severe working conditions in machinery, except for simpler shapes that can be rolled plates, profiles or welded parts, are mostly forged.

Forging can be divided into the following types according to the forming method:

① Free forging: using impact force or pressure to deform the metal between the upper and lower irons (anvils) to obtain the required forgings. There are mainly two types: manual forging and mechanical forging. The characteristic of free forging is that the metal is compressed in height and can freely extend and widen in the horizontal direction. Free forging is suitable for small batch production of large forgings with simple shapes.

② Die forging: Die forging is divided into open die forging and closed die forging. The metal billet is compressed and deformed in a forging die with a certain shape to obtain forgings. It can be divided into cold heading, roll forging, radial forging and extrusion, etc. Die forging is suitable for producing forgings with complex shapes and can be produced in large quantities.

Custom Forging can be divided into hot forging (forging temperature is higher than the recrystallization temperature of the billet metal), warm forging (forging temperature is lower than the recrystallization temperature of the metal) and cold forging (normal temperature) according to the deformation temperature. The recrystallization temperature of steel is about 460, but 800 is generally used as the dividing line. Forging above 800 is hot forging; forging between 300 and 800 is called warm forging or semi-hot forging. The defects caused by improper forging process are usually the following. 1. Large grains Large grains are usually caused by excessively high initial forging temperature and insufficient deformation, or excessively high final forging temperature, or deformation falling into the critical deformation zone. Aluminum alloys deform too much to form textures; high-temperature alloys deform too low to form mixed deformation structures, which may also cause coarse grains. Coarse grains will reduce the plasticity and toughness of forgings, and significantly reduce fatigue performance. 2. Uneven grains Uneven grains refer to the fact that the grains in some parts of the forging are particularly coarse, while those in some parts are smaller. The main reason for uneven grains is that the uneven deformation of the billet causes different degrees of grain breakage, or the deformation degree of the local area falls into the critical deformation zone, or the high-temperature alloy is locally hardened, or the local grains are coarse during quenching and heating. Heat-resistant steel and high-temperature alloys are particularly sensitive to uneven grains. Uneven grains will significantly reduce the durability and fatigue properties of forgings.

3. Cold hardening phenomenon

During deformation, due to low temperature or too fast deformation speed, as well as too fast cooling after forging, the softening caused by recrystallization may not keep up with the strengthening (hardening) caused by deformation, so that the cold deformation structure is still partially retained inside the forging after hot forging. The existence of this structure improves the strength and hardness of the forging, but reduces the plasticity and toughness. Severe cold hardening phenomenon may cause forging cracks.

4. Cracks

Cracks are usually caused by large tensile stress, shear stress or additional tensile stress during forging. The location where the crack occurs is usually the location where the billet has the largest stress and the thinnest thickness. If there are micro cracks on the surface and inside of the billet, or there are structural defects in the billet, or the hot working temperature is inappropriate, which reduces the plasticity of the material, or the deformation speed is too fast, the deformation degree is too large, and it exceeds the allowable plasticity index of the material, then cracks may occur in the processes of roughing, drawing, punching, expanding, bending and extrusion.

5. Cracking

Cracking is a shallow turtle-shaped crack on the surface of the forging. The surface subjected to tensile stress during forging (for example, unfilled protrusions or bent parts) is most likely to produce this defect. The internal causes of cracking may be multifaceted: ① The raw materials contain too many fusible elements such as Cu and Sn. ② When heated at high temperature for a long time, copper precipitates on the surface of the steel, the surface grains are coarse, decarburized, or the surface has been heated many times. ③ The sulfur content of the fuel is too high, and sulfur penetrates into the surface of the steel.

6. Flash cracks

Flash cracks are cracks generated at the parting surface during die forging and trimming. The causes of flash cracks may be: ① During the die forging operation, the metal flows strongly due to heavy impact, resulting in rib penetration. ② The trimming temperature of magnesium alloy die forgings is too low; the trimming temperature of copper alloy die forgings is too high.

7. Parting surface cracks

Parting surface cracks refer to cracks generated along the parting surface of the forging. The raw material has many non-metallic inclusions, and the flow and concentration to the parting surface during die forging or the shrinkage tube residue squeezes into the flash during die forging, often forming parting surface cracks.

8. Folding

Folding is formed when the oxidized surface metal merges together during metal deformation. It can be formed by the confluence of two (or more) metal convections; it can also be formed by the rapid and large-scale flow of a metal that carries the surface metal of the adjacent part with it, and the two merge; it can also be formed by the bending and reflow of deformed metal; it can also be formed by partial deformation of part of the metal and being pressed into another part of the metal. Folding is related to the shape of the raw materials and billets, the design of the mold, the arrangement of the forming process, the lubrication conditions and the actual operation of forging. Folding not only reduces the bearing area of the parts, but also often becomes a fatigue source due to the stress concentration here during operation.